critical facultycrit-i-cal fac-ul-ty
A good working definition would be: The ability to mentally evaluate information, statements, or propositions, to determine if they are accurate, true, or likely.
dojodo-jo
A school for training in Japanese arts of self-defense, such as judo and karate.
On The Critical Faculty:“It is our only guarantee against delusion, deception, superstition, and misapprehension of ourselves and our earthly circumstances.” William Graham Sumner

It seems replaying it and zooming in did nothing to alter that impression either.

So - could it be a sneak preview of a futre Dr Who companion?

One with the DR's special tweaks that lets cell phones work accross time?

Or maybe a member of security staff from the USS Enterprise quietly chatting with the ship on her communicator whilst it was on an adventure in the past. Who knows could the man in the hat be a shot of a heavily disguised Spock with a hat covering his ears and eyebrows...

It is not likely it could actually be a current cell phone running on the current systems. They require an extensive infrastructure to make them work, with a network of arials to allow them to connect through to the provider's exchange and route the call on to the recipient.

Still there are other possible explanations - maybe time traveler with a walkie-talkie..

Of course there is the alternative that it is just coincidence, or someone who is hard of hearing, or even a hoax, by whoever put the original DVD together, we all saw Forest Gump at famous historical events, computing has moved on since that movie was made.